Fuses with fulgurite suppressing means



United States Patent O M' FUSES WITH FULGURITE SUPPRESSING MEANS Frederick J. Kozacka, South Hampton, N.H., assignor to Chase-Shawmut Company, Newburyport, Mass., a corporation of Massachusetts Application July 24, 1958, Serial No. 750,617

Claims. (Cl. 200-120) This application is a continuation-impart of my copending patent application Ser. No. 658,162, tiled May 9, 1957, now Patent No. 2,866,038, for Current-Limiting Fuses With Increased interrupting Capacity.

The above application is concerned with means for precluding shunting of arc gaps by hot fulgurites having a relatively high conductivity. To achieve this end the fuse link is sandwiched between a pair of arc-chute-forming plates which comprise a silicious heat-absorbing material in finely divided form, e.g. glass fibers, and an organic synthetic resin binder for said material. Such plates have a very high heat-absorbing capacity, comparable to that of a quartz sand filler and tend, therefore, to result in rapid current decay following arc inception. The synthetic resin binder of such plates precludes such plates from being converted under the heat of the arc into a relatively dense fulgurite having a relatively high conductivity When and as long as hot.

The particular structure disclosed in the above patent application lends itself well to the construction of current-limiting fuses for relatively small circuit voltages, e.g. circuit voltages of less lthan 250 volts.

lt is, therefore, one object of this invention to provide electric fuses based on the same principle as those disclosed in the above referred-to patent application and likewise precluding the formation of arc-gap-shunting fulgun'tes which fuses lend themselves particularly Well for relatively higher circuit voltages say, circuit voltages in excess of 250 Volts.

Another object of the invention is to provide fuses having fuse links designed to form serially related breaks, or multi-breaks, which fuses have but two cover plates made of a silicious heat absorbing material in finely divided form and of a synthetic resin binder for precluding formation of arc-gap-shunting fulgurites tending to impair the effectiveness of the fuses.

Another object of the invention is -to provide fuses having fuse links covered by plates made of a silicious heat Iabsorbing material in nely divided form and of a synthetic resin bider for that material which fuses are designed in such a way that the link cover plates do not need to be relatively thin and exible but may be relatively thick and inflexible for the purpose of increasing the heat absorbing capacity thereof.

Another object of the invention is to provide fuses wherein fulgurite-formation isv concentrated along the residue of the fuse links upon partial vaporization of the same under the heat of the arc, and wherein fulgurite formation is minimized immediately -adjacent the are gaps where fulgurite formation -tends to shunt the arc gaps and to impair the effectiveness of the fuses.

Other objects and advantages of -the invention will become apparent -as this specification proceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the appended claims forming part of this specication.

For a better understanding of the invention reference Qmaybe had to the accompanying drawingin which:

i2,892,061` Patented June 23, 1959Av ICC Fig. 1 is a longitudinal section of a fuse embodying the present invention taken along 1-1 of Fig. 2;

Fig. 2 is a section of the same fuse taken along 2-2 of Fig. 1 and drawn on a larger scale than Fig. l;

Fig. 3 is a view similar to that of Fig. 1 of a detail of a modified version of a fuse embodying the present invention; and

Fig. 4 shows the same structure as Fig. 3 after blowing of the fuse under major fault conditions.

Referring now to Figs. 1 and 2, the fuse shown therein comprises a casing or fuse tube 1 of insulating material which contains a pulverulent fulgurite-forming arcquenching ller 2 such as quartz sand. Both ends of fuse tube 1 are closed by caps 3 of sheet metal and circular gaskets 4 are interposed between the ends of fuse tube 1 and caps 3. Knife-blade contacts 5 project transversely through caps 3, and pins 6 project transversely through caps 3 and knife-blade contacts 5 and secure both these parts to fuse tube 1, The axially inner ends of caps 3 may be crimped into fuse tube 1, if deemed necessary or desirable. The fuse link interconnects conductively the two axially inner ends of knife-blade contacts 5. To this end link 7 is spot-welded to knife-blade contacts 5 at points indicated by reference numeral 8. Fuse link 7 has a plurality of points 9 of reduced cross-section of which each is formed by a pair of lateral substantially V-shaped incisions in fuse link 7. Fuse link 7 is made of a relatively high conductivity relatively low fusing energy metal, i.e. silver or copper. Link 7 is substantially rectangular, its length b being a multiple of its width a. The fuse further comprises a pair of rectangular plates 10 each arranged on a different side of, and in immediate heat-exchanging relation with, fuse link 7. Plates 10 cover the points 9 of reduced cross-section of link 7 and define restricted paths for the escape of products of arcing from points 9 in substantially transverse direction. Normally plates 10 are flat and closely hug the surfaces of link 7. Fastener means 1l in the form of rivets, eyelets, or the like, project across both plates 10 and `across link 7 sandwiched therebetween. Fastener means 11 are arranged along the longitudinal axis of fuse link 7 and along the longitudinal axes of the pair of plates 10 and provide for `a limited freedom of motion of the longitudinal edges 12 of plates 10 at right angles to the general plane dened by plates 10 in the unbent condtion thereof, ln other Words, the presence of rivets 11 or the like is limited to the region of the common longitudinal plane of symmetry of link 7 and plates 10.

Plates 10 are made or formed of a librous silicious heat-absorbing material, such as glass cloth, embedded in -a synthetic resin binder, such as a silicone resin binder,

or a melamine binder. Plates 10 are rectangular and the width a of plates 10 exceeds the Width a of link 7, but the length b of plates ltr is less than the length b of link 7. Therefore the lateral or longitudinal edges 12 of plates 10 project beyond the lateral or longitudinal edges 13 of link 7, thus tending to keep the quartz sand tiller 2 away from the region of fuse link 7. Therends of fuse link 7 project, however, beyond the ends of plates 10, thus making it readily possible to attach the,

yas heat-absorbers, their contact with link 7 ought to be as intimate as possible. This requirement precludes the vinterposition of any heat protective layers such as, ,for

instance, a sheet of mica, between link 7 and plates 10. It is, therefore, necessary in order to avoid damage to the synthetic resin binder of plates l@ by the heat generated in the fuse link 7 during normal current-carrying operation of the link to minimize the normal operating temperature of the link and to use a synthetic resin. binder for plates 10 which is as heat-resistant as possible. Sili cone resins form binders for silicious substance in nely divided form which are highly heat resistant. The average operating temperature of points 9 of link 7 may be minimized by associating each point 9 with a link-destroying means of a low fusing point metal such as tin, tin alloys, indium, or indium alloys. The aforementioned link-destroying means may be formed by an overlay ot one of the aforementioned metals. lf the fuse is required t operate as fast as possible on the occurrence of major fault currents such overlays ought to be as thin as compatible with the aim of link-destruction, say M000. It the link-destroying overlay of a low fusing metal is relatively thick, a time delay is introduced by the increase of the mass of metal of the fuse link that must be brought up to fusing temperature preparatory to interruption of the circuit. Overlays of appropriate thickness may be readily produced by electroplating links "i with one of the above link-destroying low fusing point metals.

The operation of the fuse structure shown in Figs. l and 2 is as follows. On major faults initial fusion and arc initiation occurs at points 9 where current density is highest. The pressure build-up at points 9 results in substantially transverse blasts of gas indicated in Fig. 2 by a pair of arrows R. Plates l() form iiexible arc chutes delining passages for the escape of products of arcing increasing in size in proportion to the magnitude of pressure build-up at points 9. An intense cooling action takes place the very instant an are is kindled at points 9, i.e. even before an arc extinguishing blast of gas can develop, this instant cooling action being due to the cornposition of plates 10, i.e. the fact that they are made up of silica or a similar silicious substance in finely divided form as, for instance, glass fibers and a suitable synthetic resin binder for the same. Fulgurite-formation is, in substance, limited to the zone outside of plate edges 12 and thus being relatively remote from the path of the two series arclets formed immediately adjacent to points 9 in the space between the two plates 10.

As` mentioned above, the arc products escaping from the arcing zone low more or less in transverse direction, as indicated by arrows R in Fig. 2. The more or less transverse flow of products of arcing away from the arcing zone is not as advantageous as venting in a direction longitudinally of the link inasmuch as in the last mentioned instance the link forms an effective cooler for the products of arcing, and is being pre-heated by the same, thus increasing the rate of burn-back. The arrangement shown in Figs. l and 2 has, however, the important advantage that a heat-absorbing arc chute structure for any number of series arclets formed at points of reduced cross-section of a link can be made by resorting to but two heat-absorbing arc chute plates 10.

Referring now to Fig. 3, the structure shown therein comprises a fulgurite-forming pulverulent filler 2', eg. quartz sand, housed in a fuse tube of the same kind as shown in Fig. l but not shown in Fig. 3. The fuse link 7' is spot welded at 8 to knife-blade contacts 5 and sandwichedl between a pair of plates 10. Plates 1li" of Fig. 3 are identical to the plates 1t] of Fig. l, and the fuse link 7' of Fig. 3 is identical to the fuse link 7 of Fig. l. The rivets 11 shown in Fig. 3 for attaching plates lli' to the lfuse link 7 are arranged outside of the common longitudinal plane of symmetry of parts 7 and 10 and thus preclude any significant bending of the longitudinal edges 12' of plates 10'.

The structure shown in Fig. 2 has a certain current rating and a certain voltage rating as, for instance, l2() amps. at 600 volts. The nature of link 7 (the kind of metal of which it is made and its geometry and size), the nature of plates 10 (the kind of silicious substance of which plates 10 are made, the mode of its tine division and the kind of synthetic resin used as binder) and the nature of the body of quartz sand (its grain size and the kind of impurities which it may possibly contain) define a maximum burn-back length from each of the points of reduced cross-section 9. Reference letter c' has been applied in Fig. 4 to indicate this maximum burnback length. The distance e between the points 9 of reduced cross-section of link 7 exceeds the maximum burn-back length c. ln other words, the link 7 will never be entirely consumed, there will always be a link residue even under the most onerous interrupting condition which can occur with the particular fuse in a particular circuit. The minimum residual portion of link 7', i.e. the portion thereof which does not vaporize even under the most onerous interrupting conditions has clearly been shown in Fig. 4. This ligure shows also the fu1- gurites 14 which form incident to blowing of the fuse. It will be noticed that fulgurite-formation is concentrated along, or immediately adjacent to, the residual portion of the fuse link, and that fulgurite-formation is minimized immediately adjacent the loci of the points 9' of reduced cross-section of link 7'. In other words, fulgurite formation is concentrated where the 1fulgurites 14 do not cause any harm because these points are away from the points of break. There is virtually no fulgurite-formation where the circuit interrupting breaks are formed, or the fulgurites are very loose at these points and therefore have a relatively high resistivity, even if hot.

The pattern of Fig. 4 is typical of any X-ray picture taken of a fuse of the type of Fig. 3 upon blowing thereof. It is apparent from Fig. 4 that virtually no fulgurites are being formed adjacent the wide open ends of the V-shaped incisions in link 7 and that fulgurite-formation is limited to the areas of the lateral edges of the residual link portion where fulgurite-formation might least be expected.

While the two embodiments of the invention illustrated in the drawing comprise fuse links of which each has but two serially related points of reduced cross-section, it will be apparent that the invention is applicable to fuse links having any desired number of serially related points of reduced crosssection, and that the number of heat-absorbing and filler-shielding are chute plates 10 and 10' will be limited to two, irrespective of the number of points 9 and 9 of reduced cross-section provided along the fuse link.

Inasmuch as the structure of Figs. 3 and 4 is not predicated on bending of plates 10 to form arc chutes having variable venting passages as in the case of Figs. 1 and 2, any desired thickness may be given to plates 10' and thus their heat-absorbing capacity increased, as de'- sired.

It will be apparent from the foregoing that in both embodiments of the invention shown plates 10 and 10', respectively, are adapted to define venting passages limiting the escape of the products of arcing from the points of reduced cross-section 9 and 9', respectively, substantially to the plane defined by the fuse link and to preponderantly transverse direction.

I am not fully aware of the theoretical reasons underlying the mode of operation of the structure of Fig. 3 as evidenced by the peculiar fulgurite-forrnation shown in Fig. 4. It would seem that the formation of fulgurite is relatively small, or insignificant, where the velocity of the blast of the arc products escaping from the loci of the points 9 of reduced cross-section is highest because high blast velocity minimizes the time of heat exchange between the blast and the particles of quartz sand over which the blast sweeps.. Where the relative velocity between the blast of arc products and the particles of quartz over which the are products sweep is relatively small, the time of heat exchange is relatively large and therefore the fusion of the particles of quartz sand into a fulgurite is more complete. Where the blast intensity is high and the particles of quartz sand are but slightly sintered together, the mechanical impact of the blast upon the particles will tend to cause relative motion of the particles, i.e. tend to inhibit fulgurite-formation.

It will be understood that though but two embodiments of the invention have been illustrated and described in detail, the invention is not limited thereto. It will also be understood that the structures illustrated may be modified without departing from the spirit and scope of the invention as set forth in the accompanying claims.

I claim as my invention:

l. An electric fuse comprising in combination a fuse link having a plurality of points of reduced cross-section spaced in a direction longitudinally of said link; link destroying means of a low fusing point metal associated with each ofsaid plurality of points of reduced crosssection; a pair of plates comprising silicious material in finely divided form and an organic synthetic resin binder for said material, each of said pair of plates being arranged at a different side of and in immediate heat-exchanging relation with said link, said pair of plates covering said plurality of points of reduced cross-section and being adapted to define venting passages limiting the escape of products of arcingv from said plurality of points of reduced cross-section substantially to the plane defined by said link and to preponderantly transverse direction; means for securing said pair of plates to said link; and a silicious pulverulent fulgurite-forming substance submersing said 'pair of plates and being substantially separated by said pair of plates from said plurality of points of reduced cross-section.

2. An electric fuse comprising in combination a fuse link formed of a rectangular metal ribbon having a predetermined width and a predetermined length exceeding said width, said link having a plurality of serially related points of reduced cross-section; link-destroying means of a low fusing point metal associated with each of said plurality of pointsof reduced cross-section; a pair of rectangular plates formed of a fibrous silicious heat absorbing material embedded in a binder of synthetic resin, said pair of plates having a width exceeding said predetermined with of said `link and said pair of plates having a length less than said predetermined length of said link, said pair of plates `sandwiching therebetween a portion of said link including said plurality of points of reduced cross-section thereof and being adapted to define venting passages limiting ythe escape of products of arcing from said plurality of points of reduced cross-section substantially to the plane defined by said link and to preponderantly transverse direction; fastener means for securing said pair of plates to said link; and a body of quartz sand in which said pair of plates is submersed.

3. An electric fuse comprising in combination a ribbon fuse link of a relatively high conductivity relatively low fusing ener-gy metal having a plurality of serially related points of reduced cross-section; link-destroying means of a low-fusing point metal associated with each of said plurality of points of reduced cross-section; a pair of plates formed of a fibrous silicious heat-absorbing material embedded in an organic binder, each of said pair of plates being arranged at a different side of and in immediate heat-exchanging relation with said link and each of said pair of plates covering said plurality of points of reduced cross-section; fastener means for securing said pair of plates to said link, said fastener means being arranged along the longitudinal axis of said link and along the longitudinal axes of said pair of plates only and providing a limited freedom of motion to the longitudinal edges of said pair of plates at right angles to the general plane of said pair of plates; and a silicious pulverulent substance submersing said pair of plates and 6 being substantially separated by said pair of plates from said plurality of points of reduced cross-section.

4. An electric fuse comprising in combination a ribbon fuse link of silver having a plurality of serially related points of reduced cross-section; link-destroying means of a low fusing point metal associated with each of said plurality of points of reduced cross-section; a casing of insulating material housing said link; a pair of terminal elements closing said casing conductively interconnected by said link; a pair of plates formed of a fibrous silicious heat-absorbing material embedded in an organic binder, each of said pair of -plates being arranged ata different side of and in immediate heat-exchanging relation with said link and each of said pair of plates covering said plurality of points of reduced crosssection; fastener means projecting across said link and said pair of plates for securing said pair of plates to said link, the presence of said fastener means being limited to the -region of the common longitudinal plane of symmetry of said link and of said pair of plates; and a body of quartz sand inside said casing submersing said pair of plates.

5. An electric fuse comprising in combination a ribbon fuse link formed of a rectangular metal ribbon having a predetermined width and a predetermined length exceeding said predetermined width, said link having a plurality of serially related points of reduced cross-section; a fuse tube of insulating material housing said link; a pair of terminal elements closing said fuse tube conductively interconnected by said link; a pair of rectangular plates formed of a brous silicious heat-absorbing material embedded in a synthetic resin binder, said pair of plates having a width exceeding said predetermined width of said link and said pair of plates having a length less than said predetermined length of said link, said pair of plates sandwichingtherebetween a portion of said link including said plurality of points of reduced crosssection thereof; a plurality of aligned fasteners limited to the region of the common longitudinal plane of symmetry of said link and said pair of plates for securing said pair of plates to said link; and a body of quartz sand inside said fuse tube submersing said pair of plates.

6. An electric fuse having a predetermined voltage rating and a predetermined current rating comprising in combination a ribbon fuse link of a relatively high conductivity relatively low fusing energy metal having a plurality of points of reduced cross-section spaced a predetermined distance in a direction longitudinally of said link; link-destroying means of a low fusing point metal associated with each of said plurality of points of reducedcross-section; a pair of plates formed of silicious heat-absorbingv material in finely divided form embedded in a synthetic resin binder, each of said pair of plates being arranged lat a different side of and in immediate heat exchanging relation with said link and covering said plurality of points of reduced cross-section thereof; a silicious pulverulent fulgurite-forming ller submersing said pair of plates and being substantially separated by said pair of plates from said plurality of points of reduced cross-section of said link; the voltage rating and current rating of said fuse and the nature of said link, the nature of said pair of plates and the nature of said filler jointly defining a maximum burn-back length from each of said plurality of points of reduced cross-section; and said predetermined distance between said plurality of points of reduced cross-section of said link exceeding said burn-back length whereby upon blowing of the fuse fulgurite-formation is concentrated at the residual portion of said link and fulgurite-formation is minimized at the loci immediately adjacent said plurality of points of reduced cross-section thereof.

7. An electric fuse having a predetermined voltage rating and a predetermined current rating comprising in combination a fuse link formed of a rectangular metal ribbon having a predetermined width and a predetermined length exceeding said width, said link havinga plurality of points of reduced cross-section spaced a predetermined distance in a direction longitudinally of said link; a link-destroying overlay of a low fusing point metal associated with each of said plurality of points of reduced cross-section; a pair of rectangular plates formed of silicious material in finely divided form and a synthetic resin binder for said material, said pair of plates having a width exceeding said predetermined width of said link and said pair of plates having a length less than said predetermined length of said link, said pair of plates sandwiching therebetween a portion of said link including said plurality ofpoints of reduced cross-section thereof, and said pair of plates being adapted to deiine venting passages limiting the escape of products of arcirig from said plurality of points of reduced cross-section substantially to the plane defined by said link and to preponderantly transverse direction; means for securing said pair of plates to said link; a body of quartz sand submersing said pair of plates; the voltage rating and current rating of said fuse and the nature of said link, the nature of said pair of plates and the nature of said body of quartz sand jointly defining a maximum burn-back length from each of said plurality of points of reduced crosssection; and said predetermined distance between said plurality of points of reduced cross-section and said link exceeding said burn-back distance whereby upon blowing of the fuse fulgurite-formation is concentrated immediately adjacent the residual portion of said link and fulgurite-formation is minimized at the loci immediately adjacent said plurality of points of reduced cross-section thereof.

, 8. A fuse as specified in claim 7 wherein said plurality of points of reduced cross-section are formed by a plurality of pairs of lateral substantially V-shaped incisions into said link,

v 9. An electric fuse having a predetermined voltage rating and a predetermined current rating comprising in combination a ribbon fuse link having a plurality of points of reduced cross-section spaced a predetermined distance in a direction longitudinally of said link; a linkdestroying overlay of a low fusing point metal associated with each of said plurality of points of reduced crosssection; a tubular casing of insulating material housing said link; a pair of terminal elements closing said casing conductively interconnected by said link; a pair of plates formed of a fibrous silicious heat absorbing material embedded in a binder of synthetic resin, each of said pair of plates being arranged at a different side of and in immediate heat-exchanging relation with said link and each of said pair of plates covering said plurality of points of reduced cross-section and being adapted to define venting passageslimiting the escape of products of areing from said plurality of points of reduced cross-section substantially to the plane deiined by said link and to preponderantly transverse direction; a body of quartz sand inside of said casing submersing said pair of plates; the voltage rating and current rating of said fuse and the nature of said link, the nature of said pair of plates and the nature of said body of quartz sand jointly defining a maximum burn-back length from each of said plurality of points of reduced cross-section; and said predetermined distance between said plurality of points of reduced cross-section of said link exceeding said burn-back length whereby upon blowing of the fuse fulgurite formation is concentrated immediately adjacent to the residual portion of said link and fulgurite-formation is minimized at the loci immediately adjacent of said plurality of points of reduced cross-section thereof.

l0. An electric fuse having a predetermined voltage rating and a predetermined current-rating comprising in combination a fuse link formed of a rectangular metal ribbon having a predeterminedv width and a predetermined length exceeding said Width, said link having a plurality of points of reduced crosssection spaced a predetermined distance in a direction longitudinally of said link; a link destroying overlay of a low fusing point metal associated with each of said plurality of points of reduced cross-'section of said link; a tubular casing of insulating material housing said link; a pair of terminal elements closing said casing conductively interconnected by said link;` a pair of rectangular plates formed of fibrous silicious heat-absorbing material embedded in a synthetic resin binder', said pair of plates having a width exceeding said predetermined width of said link and sandwiching therebetween a portion of said link including said plurality of points of reduced cross-section thereof; a plurality of fasteners for securing said pair of plates to said link; a fulgurite-for'ming pulve'rulent ller inside of said casing submersing said pair of plates; the voltage rating and current rating of said fuse and the nature of said link, the nature of said pair of plates and the nature of said filler jointly dening amaximum burnback length from each of said plurality of points of reduced cross-section; and said predetermined distance between lsaid plurality of points of reduced cross-section of said link exceeding said burn-back distance whereby upon blowing of the fuse fulgurite-for'mation is concentrated immediately adjacent to the residual portion of said link and fulgurite-formation isy minimizedv at the loci immediately adjacent of said plurality of points of reduced cross-section thereof.

References Cited in the le of this patent UNITED STATES PATENTS 2,018,556 Hope Oct. 22', 1935 2,808,487 Jacobs v 1 Oct. 1, 1957 2,833,890 Jacobs May 6, 1958 

